Comparative evaluation of the MGIT and BACTEC systems for the culture of Mycobacterium avium subsp. paratuberculosis from milk

The non-radiometric Mycobacterium Growth Indicator Tube (MGIT) culture system has not previously been evaluated for the culture of Mycobacterium avium subsp. paratuberculosis (Map) from milk. A study was undertaken to compare the detection capabilities and detection times of the MGIT system with those of the radiometric BACTEC 460 system currently used in our laboratory. UHT milk samples were spiked with a range of Map concentrations (10-10 cells/ml). MGIT tubes supplemented with MGIT OADC, MGIT PANTA and mycobactin J, and BACTEC vials supplemented with PANTA and mycobactin J, were inoculated with 500 μl of triplicate milk samples at each Map concentration. MGIT tubes were read manually using a UV transilluminator and BACTEC vials on the BACTEC 460TB machine. Time to detection of growth in days was recorded for each system. A corresponding BACTEC count was determined from the BACTEC growth index readings using a published formula. A further experiment assessed the recovery of Map from milk by both culture systems following decontamination with 0.75 % (w/v) cetylpyridinium chloride for 5 h. In the absence of any decontamination step both the MGIT and BACTEC culture systems were capable of detecting growth from milk samples containing 10100 Map cells within 30-40 d. The correlation coefficient between MGIT and BACTEC detection times was 0.826. MGIT detection times tended to be shorter than BACTEC detection times when low numbers of Map were present in a milk sample and slightly longer when high numbers of Map were present. After decontamination only milk samples originally spiked with > 10-10 Map exhibited growth in both culture systems. Decontamination caused a significant reduction (mean 1.44 log) in numbers of viable Map in all spiked milk samples. Overall, the MGIT system was shown to have similar detection capabilities to the BACTEC system for recovering Map from milk.